The present invention relates to a control system for a vehicle with at least part-time four-wheel drive. More particularly, the present invention relates to a control system for a vehicle with at least part-time four-wheel drive, having a control unit which detects or determines quantities proportional to the wheel speeds of all wheels and the vehicle speed, and by means of which the driving torque of a drive unit can be variably distributed to primary driving wheels, which are permanently connected with the drive unit, and to secondary driving wheels which, if required, can be connected with the drive unit.
Control systems of this type are described, for example, in German Patent Document DE 100 54 023 A1. Accordingly, a torque distribution device is known for changing the torque distribution ratio between the wheels of the front axle and the wheels of the rear axle by a corresponding control of a friction clutch as a longitudinal blocking device (transfer clutch). The driving action of a vehicle can be influenced considerably by the determination of a torque distribution ratio. In particular, the object of German Patent Document DE 100 54 023 A1 relates to the driving dynamics when cornering. In this case, a partially four-wheel-driven motor vehicle may be a basically front-wheel-driven motor vehicle with a rear-wheel drive which can be connected by means of a transfer clutch; a basically rear-wheel-driven motor vehicle with a front-wheel drive which can be connected by way of a transfer clutch; or a permanent all-wheel drive vehicle with a controllable transfer clutch for changing the torque distribution between the front and rear axle.
Generally, as described below, primary driving wheels are the wheels which are permanently connected with the drive unit, and secondary driving wheels are the wheels which, if required, can be connected with the drive unit by way of the transfer clutch.
Furthermore, such known control systems usually have a control of the transfer clutch as a function of the rotational speed difference between a rotational speed of the primary driving axle and the rotational speed of the secondary driving axle (for example, German Patent Documents DE 37 41 009 A1, DE 69304449 T2). In this case, the rotational speeds of the driving axles are detected, for example, by means of separate sensors.
It is an object of the present invention to improve a control system of the previously mentioned type with respect to the control.
The present invention is based on the recognition that, on the one hand, additional rotational speed sensors on the driving axles are cost-intensive and that the regulating of a slip between the drive shafts is too inaccurate with respect to the controlling of the driving dynamics. The precision of the slip control process can be coupled if, as a function of an individual wheel slip, it takes place relative to all four wheels or, as a function of a wheel slip between the individual wheels, it takes place on one (longitudinal) vehicle side respectively.
According to exemplary embodiments of the present invention, in the case of a control system for a vehicle with at least part-time four-wheel drive, by means of a control unit, which detects or determines quantities proportional to the wheel speeds of all wheels and the vehicle speed, and as a result of which the driving torque of a drive unit can be variably distributed to primary driving wheels which are permanently connected with the drive unit, and to secondary driving wheels which, if required, can be connected with the drive unit, the adjusting unit of a transfer clutch arranged between the drive unit and the secondary driving wheels is, therefore, controlled such that the clutch torque and thus the driving torque onto the secondary driving wheels is increased, if a difference is detected between the wheel speed of one wheel and the vehicle speed and/or if a difference is detected between the two wheel speeds of the wheels of at least one (longitudinal) vehicle side (left, right in the driving direction).
The driving torque is the output torque of a drive unit which may consist of an internal-combustion engine and a transmission following it. For this purpose, the control unit defines a wheel-slip-dependent desired clutch torque for increasing the clutch torque, which desired clutch torque is determined as a function of the amount of the difference between the wheel speed of one wheel and the vehicle speed and/or as a function of the amount of the difference between the two wheel speeds of the wheels of at least one vehicle side.
The quantity proportional to the wheel speed of a wheel may be a rotational wheel speed, which is detected and which, in a known manner, can be converted into a wheel speed in meters per second (v) which is directly comparable with the vehicle speed.
The wheel-slip-dependent desired clutch torque may be determined in the control unit by a maximal selection from at least three desired clutch torques: a first desired clutch torque which is determined as a function of the amount of the difference between the wheel speed of a wheel and the vehicle speed; a second desired clutch torque which is determined as a function of the amount of the difference between the two wheel speeds of the wheels of the left vehicle side; and a third desired clutch torque which is determined as a function of the amount of the difference between the two wheel speeds of the wheels of the right vehicle side.
An advantage of the individual wheel slip taken into account by the present invention is the sensitivity, which is twice as high as in the state of the art, by means of which the torque distribution can be controlled. This is particularly advantageous when cornering, because usually only the wheels on the inside of the curve tend to spin. As a result, by means of the invention, an increase of the traction characteristics and of the stability characteristics of the vehicle may be achieved.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.